This invention relates to the field of keypad illumination and particularly, to the application of electroluminescent systems for this purpose.
Keypads are widely used to facilitate data and command input for variety of devices in the consumer electronics industries. These devices include cellular telephones, personal organizers, hand-held computers, pagers, calculators and the like. The design of such keypads has been refined to provide good tactile feedback in light-weight, inexpensive sub-assemblies. When there is a potential for use under low ambient lighting it is desirable to provide integral keypad illumination for some devices. For example, keypad illumination can be particularly useful in cellular telephones and personal organizers. In the current state-of-the-art, such illumination is provided by an array of subminiature or surface mount LED lamps, nested in a minimal space between the keypad and the key contact panel. While such arrangements can be effective, power consumption and bulk pose inherent design challenges. Power consumption is sometimes addressed by inclusion of a power-on time limit. The bulk effect can be minimized by careful design, but most likely at the expense of uniform illumination. Thus, there are trade-offs and compromises that can impact both functionality and sales appeal of the product.
Therefore, a first object of the present inventions is to provide apparatus that facilitates the illumination of keypads. A second object of the inventions is that this keypad illumination apparatus be in a compact, light-weight form that does not materially increase product bulk and/or weight. A third object of these inventions is to provide keypad illumination apparatus with a significantly reduced power requirement. Yet other objects are that this apparatus maintains good tactile feel and provides uniform light distribution.
In a preferred embodiment of the present inventions apparatus for back-lit illumination of key designations in a keypad comprises a plurality of individual, translucent key cores. A base membrane disposed in a horizontal plane around and between key cores maintains the key pattern. A thin flexible, opaque film covers the upper surface of the base membrane and the key cores and is laser etched to expose characters for illumination on the key operating ends. In the alternative, the opaque film covering may be omitted and the characters may be opaque. An elastomeric, membrane-like electroluminescent lamp (EEL), as disclosed by Burrows in U.S. Pat. No. 5,856,030, is interposed between the keypad and the key contact panel where it is held in assembly against the horizontal lower surface of the base membrane and the lower, actuating ends of the keys, so as to be stretched when a key is actuated. Positioned thusly, the EEL serves to illuminate the designation characters through the key cores when activated.